The thymus is the key primary lymphoid organ responsible for the generation of T cells, which provides the necessary signals and cellular environment for their survival and differentiation. Analysis of the fetal origins, developmental pathways, and molecular and cellular mechanisms controlling thymus organogenesis have provided key insights into understanding postnatal thymus function. Recently, the existence and initial characterization of a second, cervical thymus has been reported in both mouse and humans. These studies showed that the cervical thymus is a site of production for functional T cells, which can contribute to peripheral immunity. While it is clear that cervical thymus tissue exists and can generate T cells, its ontogeny remains to be elucidated. Data from these recent reports and from our lab indicate that the cervical thymus in mice is not identifiable by Foxn1 gene expression before the newborn stage, and is frequently associated with the parathyroid and/or thyroid glands. The location and timing of its appearance raise the questions of the cellular origins of the cervical thymus, the molecular mechanisms controlling its development, and what induces its postnatal appearance. Analysis of the mechanisms underlying cervical thymus formation provides a unique opportunity to understand the cellular and molecular requirements for de novo thymus formation. The variable appearance of cervical thymi in both mice and people begs the question of whether it is the potential to generate cervical thymi (i.e. presence of the progenitor cells), or the ability to stimulate their development that is variable. If it is the latter, this raises the intriguing possibility that latent thymic potential may exist in everyone, and thus may represent a novel therapeutic target for thymic regeneration and rebound. Understanding of the ontogeny and regulation of cervical thymus development is therefore critical to any future studies of the function of this organ in normal immunity, disease response, and age-related involution. In the current proposal, we will test the hypothesis that the cervical thymus arises from the same 3rd pouch derived primordium and is regulated by the same molecular mechanisms as the parathyroids and thoracic thymus. In the past 10 years, my lab has identified multiple transcription factor and signaling pathways that control the patterning and early organogenesis of the thoracic thymus and parathyroids from the 3rd pharyngeal pouch, and defined the morphological events underlying the early development of these organs, including the origin of the thymic epithelium. In the current proposal, we will use our extensive track record in the analysis of thymus and parathyroid organogenesis, using the tools and knowledge from our previous studies to investigate the mechanisms controlling the ontogeny of the cervical thymus in two Specific Aims: 1. Identify the origin of the epithelial cells in the cervical thymus in mice, to test the hypothesis that they originate from the endodermal organ primordia that give rise to the parathyroids and thoracic thymus; 2: Test whether the cervical and thoracic thymus depend on common molecular mechanisms and/or cellular interactions for their development. The thymus is the key primary lymphoid organ responsible for the generation of T cells, and is therefore critical for the establishment and maintenance of a functioning immune system. Recently, the existence and initial characterization of a second, cervical thymus has been reported in both mouse and humans. These studies showed that the cervical thymus is a site of production for functional T cells. The variable appearance of cervical thymi in both mice and people begs the question of whether it is the potential to generate cervical thymi (i.e. presence of the progenitor cells), or the ability to stimulate their development that is variable. If it is the latter, this raises the intriguing possibility that latent thymic potential may exist in everyone, and thus may represent a novel therapeutic target for thymic regeneration and rebound. [unreadable] [unreadable] [unreadable]